1. Field of the Invention
The present invention relates to lightning arresters, and in particular, to excess voltage arresters capable of protecting a plurality of electrical signal lines.
2. Description of the Prior Art
The prior art abounds with lightning arresters, which generally are very bulky and may be utilized for only the protection of one or two lines. Typical of this type of gas filled arrester is the U.S. Pat. No. 3,289,027 issued to R. D. Jones on Nov. 29, 1966.
Attempts to provide a multi-element surge arrester to protect equipment connected to a plurality of lines, e.g. the input and output lines coupled to a repeater or other multi-terminal intermediate equipment which has both input and output circuits, are exemplified by U.S. Pat. No. 4,074,338 issued to F. L. Simokat on Feb. 14, 1978. This type of device tends to be rather bulky and often inconvenient for mounting into equipment due to its elongated shape.
A potentially more compact type of multi-element surge arrester is disclosed in U.S. Pat. No. 3,312,868 issued to V. W. Vodicka on Apr. 4, 1967. However, the type of construction disclosed therein utilizes a conducting metal electrode disposed within a ceramic insulator. The metallic electrode is spaced from a ground metallic terminal forming the protective spark gap. When an electrical surge voltage such as lightning occurs, the excessive current is carried to the metallic electrode, across the spark gap to the ground terminal. The excessive current flowing in the metal electrodes generates heat therein, causing them to expand. Since the coefficient of expansion of the metal electrode and the ceramic insulator, into which it is mounted is dissimilar, fracturing of the ceramic insulator occurs, thereby breaking the seal originally provided between the metal electrode and the ceramic insulator permitting the gas within the housing to escape or air to enter into the housing. Although this rupture may be very minute in size, and not visible to the eye, the effectiveness of the surge arrester is destroyed and further electrical surges occurring on the transmission line will not be effectively handled by the damaged arrester, thereby causing damage to the equipment it was designed to protect.
Even if the arrester is constructed in the forms disclosed and is provided with metal electrodes with expansion coefficients closely matched to that of the cup shaped insulator, it is not consistently practicable to ensure that fracture of the insulator at or near the seals will not occur when high current discharges are passed through the electrodes and spark gaps.
An object of the present invention is to provide a multi-element arrester assembly particularly suitable for protecting sensitive equipment which is connected to a plurality of lines, e.g. the input and output of a repeater.
A further object of the present invention is to provide spark gaps across input and output lines, between input and output lines and between each line wire and ground in a common chamber, in which the gaps may be controlled in manufacture to have similar or differing breakdown voltage values according to the protection levels required for the particular equipment being protected.
Yet a further object of the present invention is to provide an arrester assembly with current carrying electrodes arranged to minimize heat transfer to the seal areas.
The present invention overcomes the shortcomings found in the prior art by providing a unique construction which utilizes a relatively small space and is readily mountable proximate the electronic equipment it is designed to protect.